1. Field of the Invention
The present invention generally relates to electron beam physical vapor deposition (EBPVD) processes and equipment. More particularly, this invention relates to such an EBPVD apparatus and process for depositing ceramic coatings, in which process temperatures are controlled in a manner that reduces the thermal conductivity of the deposited coatings.
2. Description of the Related Art
Components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor, are often provided with thermal barrier coatings (TBC) to minimize their service temperatures. To be effective, thermal barrier coatings must have low thermal conductivity and adhere well to the component surface. Various ceramic materials have been employed as the TBC, with zirconia (ZrO2) stabilized by about seven weight percent yttria (Y2O3), or 7% YSZ, being widely employed because of its ability to be readily deposited by plasma spraying and vapor deposition techniques. An example of the latter is electron beam physical vapor deposition (EBPVD), which produces a TBC having a columnar grain structure that is able to expand with its underlying substrate without causing damaging stresses that lead to spallation. Adhesion of the TBC to the component can be further enhanced by the presence of a metallic bond coat, such as a diffusion aluminide or an oxidation-resistant resistant alloy such as MCrAIY, where M is iron, cobalt and/or nickel.
Processes for producing TBC by EBPVD generally entail preheating a component, typically to about 1100xc2x0 C., and then placing the component in a heated coating chamber maintained at a subatmospheric pressure, often about 0.005 mbar. The component is supported in proximity to an ingot of the desired coating material (e.g., YSZ), and an electron beam is projected onto the ingot so as to melt the surface of the ingot and produce a vapor of the coating material that deposits onto the component. Temperature ranges within which EBPVD processes can be performed depend in part on the component and coating materials. Minimum process temperatures are generally established to ensure the coating material will suitably evaporate and deposit on the component to form a coating that exhibits adequate thermal fatigue properties, while maximum process temperatures are typically limited by the requirement to avoid microstructural damage to the component material. For YSZ deposited on nickel-base superalloys, a suitable temperature range is about 925xc2x0 C. to about 1125xc2x0 C. A stable component surface temperature promotes the desired columnar grain structure for the TBC.
Advanced EBPVD apparatuses permit batch coating operations, in which coated components are removed from the coating chamber and preheated uncoated components are then placed in the coating chamber without shutting down the apparatus, so that a continuous operation is achieved. The continuous operation of the apparatus during this time can be termed a xe2x80x9ccampaign,xe2x80x9d with greater numbers of components successfully coated during the campaign corresponding to greater processing and economic efficiencies. An example of a particularly efficient EBPVD apparatus is disclosed in commonly-assigned U.S. patent application Ser. No. 09/624,809 to Bruce et al. Throughout the deposition process, the temperature within an EBPVD coating chamber continues to rise as a result of the electron beam and the presence of a molten pool of the coating material. For this reason, EBPVD coating processes are often initiated near the targeted minimum process temperature and then terminated when the coating chamber nears the maximum process temperature, at which time the coating chamber is cooled and cleaned to remove coating material that has deposited on the interior walls of the coating chamber.
A suitable thickness for a TBC is dependent in part on the thermal conductivity of the TBC material. While greater thicknesses are more thermally protective of the underlying substrate, the amount of TBC deposited on a component must often be limited to minimize weight, particularly for rotating components of gas turbine engines. Various approaches have been proposed for minimizing thermal conductivities of TBC""s to allow for the use of thinner coatings without sacrificing thermal protection. For example,. commonly-assigned U.S. Pat. No. 6,447,854 to Rigney et al. discloses an EBPVD process in which the coating chamber is maintained at a pressure as high as about 0.020 mbar to produce a TBC with reduced thermal conductivity. Commonly-assigned U.S. Pat. No. 6,342,278 to Rigney et al. discloses an EBPVD process for depositing TBC materials with reduced thermal conductivities, attributed to the coating chamber being maintained at pressures of about 0.010 mbar or more with an oxygen partial pressure of greater than 50%, preferably at or close to 100%.
In addition to an initially low thermal conductivity, it is important that the thermal conductivity of a TBC remain low throughout the life of the component on which it is deposited. However, thermal conductivities of TBC materials such as YSZ have been observed to increase by 30% or more over time when subjected to the high temperatures within a gas turbine engine. This increase has been associated with microstructural instability, including coarsening of the zirconia-based microstructure through grain and pore growth and grain boundary creep. To compensate for this phenomenon, TBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
The present invention is a method of operating an EBPVD apparatus to deposit a ceramic coating (e.g., TBC) on an article, such that the thermal conductivity of the coating is both minimized and stabilized. More particularly, the EBPVD apparatus is operated so that the surface temperatures of the articles being coated do not exceed about 1000xc2x0 C. during deposition of the ceramic coating on the second article. According to one aspect of the invention, ceramic coatings deposited at such relatively low temperatures exhibit lower and, surprisingly, more stable thermal conductivities.
The method of this invention generally entails operating an EBPVD apparatus to perform multiple successive coating operations, which together constitute a coating campaign. The method comprises performing a first coating operation during which one or more articles are placed in a coating chamber in which a subatmospheric pressure is maintained, and an electron beam gun is operated to project an electron beam into the coating chamber and onto a ceramic material, with the electron beam heating, melting and evaporating the ceramic material to deposit a ceramic coating on the articles. The first coating operation is carried out so that the surface temperatures of the articles do not exceed about 1000xc2x0 C. during deposition of the ceramic coating. The procedure is then repeated for one or more subsequent coating operations, throughout which surface temperatures of the articles being coated remain at or below about 1000xc2x0 C.
To maintain such low temperature throughout a coating campaign, steps are taken so that the combined heat transfer to the articles processed in the subsequent coating operation occurs at a heat transfer rate that is lower than the heat transfer rate for the articles processed in the first coating operation. The lower heat transfer rate serves to maintain the surface temperatures of the articles at or below about 1000xc2x0 C. during deposition of the ceramic coating, even though the temperature within the coating chamber, attributable at least in part to thermal radiation from the coating chamber and the molten ceramic material, continuously rises during successive coating operations. By maintaining surface temperatures at or below about 1000xc2x0 C., the deposited ceramic coatings have been shown to exhibit lower thermal conductivities, on the order of about 20% lower, that remain stable when subjected to high temperatures, such as found in the hot gas path of a gas turbine engine.
Other objects and advantages of this invention will be better appreciated from the following detailed description.